1. Field of the Invention
The present invention relates to a hydraulic drive unit for driving a fan or the like.
2. Description of the Related Art
A radiator for the engine of construction machines and the like is cooled by a hydraulically operated fan. The hydraulically operated fan has a hydraulic pump as a hydraulic source and is rotated as a hydraulic motor is driven to rotate. The hydraulic pump is driven by the engine.
Lately, there are demands for operation of construction machines at a low noise level. Therefore, it is necessary to drive the hydraulically operated fan at a lower speed while securing adequate cooling performance.
To achieve it, it is necessary to control a flow rate flowing into the hydraulic motor so as to obtain the control characteristic LN3 of FIG. 9. The engine speed increases when controlled according to the control characteristic LN3, and the flow rate flowing into the hydraulic motor is kept at a fixed level when the engine speed increases and discharge flow rate Q of the hydraulic pump becomes prescribed flow rate Qc or more. Therefore, characteristic L0 is obtained, indicating that when engine speed N becomes prescribed speed Nc or more, rotational speed NF of the fan is kept at a prescribed level as shown in FIG. 4. Such a control characteristic is called as a flow control characteristic.
The flow control characteristic is obtained by adopting a variable-capacity hydraulic pump as the hydraulic pump and controlling a swash plate.
However, the variable-capacity hydraulic pump is generally expensive. Therefore, it is demanded to use a relatively inexpensive fixed-capacity hydraulic pump such as a gear pump to realize the flow control characteristic.
Therefore, the hydraulic circuit shown in FIG. 10 has been conventionally used.
(Related Art 1)
Specifically, as shown in FIG. 10, fixed-capacity hydraulic pump 2 such as a gear pump is driven by an unshown engine to discharge pressure oil to oil passage 7. The pressure oil discharged from the hydraulic pump 2 is supplied to hydraulic motor 1 through the oil passage 7.
Throttle 41 is disposed on the oil passage 7. The oil passage 7 is branched to oil passage 15 which is connected to an inlet port of control valve 20. An outlet port of the control valve 20 is connected to tank 3 through oil passage 16. The control valve 20 is provided with spring 20c. The upstream of the throttle 41 is connected to pilot port 20d through pilot oil passage 51. The pilot port 20d is one of two pilot ports 20d, 20e of the control valve 20 and located on the side opposite to the side where spring 20c is disposed. The downstream of the throttle 41 is connected to the pilot port 20e, which is located on the same side where the spring 20c is disposed, through pilot oil passage 52.
The structure of the control valve 20 of FIG. 10 is shown in FIG. 11. As shown in FIG. 11, the control valve 20 is a valve having a spool structure.
When it is assumed that a pressure on the upstream side of the throttle 41 is P2, a pressure on the downstream side thereof is P3, a sectional area of spool 21 is A and a spring force of the spring 20c is F, a balance of force acting on the spool 21 of the control valve 20 is ideally expressed by the following expression (1).
(P2xe2x88x92P3)xc2x7A=Fxe2x80x83xe2x80x83(1) 
Therefore, when the control valve 20 operates as indicated by the expression (1), force ((P2xe2x88x92P3)xc2x7A) which corresponds to pressure difference P2xe2x88x92P3 before and after the throttle 41 and the prescribed spring force (F) of the spring 20c are mutually balanced and therefore a flow rate of the pressure oil flowing through the throttle 41 is kept at a prescribed constant level according to the prescribed spring force, and an ideal flow control characteristic indicated by LN3 in FIG. 9 is obtained.
However, the flow rate actually flowing into the hydraulic motor does not become constant, and the characteristic indicated by LN1 is obtained, which indicates that the flow rate flowing into the hydraulic motor tends to increase according to an increase in pump discharge flow rate Q.
The reason is as follows. When the spool 21 of the control valve 20 opens to discharge the pressure oil to the tank 3 in FIG. 11, the pressure oil is discharged along streamline V which has a component parallel with the spool 21. Therefore, a force called a flow force acts on the spool 21 of the control valve 20 in the same direction as that of the spring force F. The flow force increases according to an increase in flow rate of the pressure oil passing through the throttle 41.
When it is assumed that the flow force is f, the balance of force acting on the spool 21 of the control valve 20 is indicated by the following expression (2).
(P2xe2x88x92P3)xc2x7A=F+fxe2x80x83xe2x80x83(2) 
When the control valve 20 operates according to the expression (2), the spool 21 is pushed back by the flow force f in a direction that the opening of the spool 21 is closed. Therefore, as indicated by LN1 in FIG. 9, the flow rate flowing into the hydraulic motor shows a tendency to increase according to the increase in pump discharge flow rate Q.
Thus, the related art 1 has drawbacks as described above.
(Related Art 2)
To remedy the drawbacks of the related art 1, it is tried to improve a notch shape or the like of the spool 21 so to remove the component possessed by the streamline V, which is parallel to the spool 21. A control characteristic of the related art 2 is indicated by LN2 in FIG. 9.
According to the related art 2 (characteristic LN2), the problems of the related art 1 (characteristic LN1) are improved to some extent, but the flow rate flowing into the hydraulic motor still tends to increase according to the increase in pump discharge flow rate Q. Therefore, even when the engine speed becomes the prescribed speed or more, the rotational speed of the fan continues to increase, and its noise cannot be suppressed to a prescribed level. In other words, a desired target to suppress the noise to a predetermined level when the engine speed is at a prescribed level or more cannot be achieved.
The present invention was made in view of the above circumstances and provides a low-cost and low-noise hydraulic drive unit by making it possible to realize an ideal flow control characteristic by means of inexpensive hydraulic equipment.
A first aspect of the present invention is directed to a hydraulic drive unit comprising:
a hydraulic source which increases a discharge flow rate according to an increase in rotational speed;
a throttle through which pressure oil discharged from the hydraulic source passes;
hydraulic equipment which operates upon inputting the pressure oil having passed through the throttle; and
a control valve which controls the pressure oil passing through the throttle so that the flow rate passing through the throttle becomes a prescribed level when the rotational speed becomes a prescribed level or more, wherein:
a force for canceling a flow force produced by the control valve is applied to the control valve.
Specifically, as shown in FIG. 2, it is assumed that a pressure on the upstream side of second throttle 42 is P1, a pressure on the downstream side thereof is P2 (pressure on the upstream side of the first throttle 41), a pressure on the downstream side of the first throttle 41 is P3, the sectional area of the spool 21 is A, the spring force of the spring 20c is F, and the flow force is f. Then, a balance of force acting on the spool 21 of the control valve 20 is indicated by the following expression (3).
(P1xe2x88x92P3)xc2x7A=F+fxe2x80x83xe2x80x83(3) 
Here, when it is assumed that the pressure difference P1xe2x88x92P2 before and after the second throttle 42 is xcex94P12 (see FIG. 2) to modify the expression (3), the following expression (4) is obtained.
xcex94P12xc2x7A+(P2xe2x88x92P3)xc2x7A=f+Fxe2x80x83xe2x80x83(4) 
In the expression (4), xcex94P12xc2x7A at the first term of the left-hand side indicates a force corresponding to the pressure difference xcex94P12 before and after the second throttle 42, which is applied to the control valve 20 in a direction opposite to that of the spring force F of the spring 20c and that of the flow force f.
According to the first aspect of the invention, for example, the force xcex94P12xc2x7A corresponding to the pressure difference xcex94P12 before and after the second throttle 42 is applied to the control valve 20 as a force capable of canceling the flow force f at the first term of the right-hand side of the expression (4).
The first aspect of the invention does not always require the second throttle 42 and can use different means if it is possible to apply a force, which can cancel the flow force f produced by the control valve 20, to the control valve 20.
The application of such a force to the control valve 20 changes the expression (4) to (P2xe2x88x92P3)xc2x7A=F, and there is obtained an ideal flow control characteristic indicated by LN3 in FIG. 9. Under control according to the control characteristic LN3 shown in FIG. 9, the engine speed increases and when the discharge flow rate Q of the hydraulic pump 2 becomes the prescribed flow rate Qc or more, the flow rate flowing into the hydraulic motor 1 is kept at a prescribed level. Therefore, there is obtained the characteristic L0 that the rotational speed NF of the fan 36 is kept at a prescribed level when the engine speed N becomes the prescribed speed Nc or more as shown in FIG. 4.
Therefore, according to the first aspect of the invention, the effect of suppressing noise to a prescribed level when the engine speed is at the prescribed level Nc or more can be achieved by inexpensive hydraulic equipment such as the hydraulic source 2 (fixed-capacity hydraulic pump 2), the control valve 20 (changeover valve 20) and the throttle 42.
A second aspect of the invention is directed to the hydraulic drive unit according to the first aspect of the invention, wherein a throttle for adjusting the flow force, which produces a pressure difference corresponding to the flow force, is disposed, and a force corresponding to the pressure difference before and after the flow force adjustment throttle is applied to the control valve.
The second aspect of the invention applies the force xcex94P12xc2x7A corresponding to the pressure difference xcex94P12 before and after the second throttle 42 for adjusting the flow force to the control valve 20 to cancel the flow force f of the first term of the right-hand side of the expression (4).
A third aspect of the invention is directed to a hydraulic drive unit comprising:
a hydraulic source which increases a discharge flow rate according to an increase in rotational speed;
a first throttle through which pressure oil discharged from the hydraulic source passes;
hydraulic equipment which operates upon inputting the pressure oil having passed through the first throttle; and
a control valve which controls the pressure oil passing through the first throttle so that the flow rate passing through the first throttle becomes a prescribed level when the rotational speed becomes a prescribed level or more, wherein:
a second throttle which produces a pressure difference corresponding to a force for canceling the flow force produced by the control valve is disposed; and
a force corresponding to the pressure difference before and after the second throttle is applied to the control valve in a direction to cancel the flow force.
The third aspect of the invention applies the force xcex94P12xc2x7A according to the pressure difference xcex94P12 before and after the second throttle 42 to the control valve 20 in a direction opposite to that of the flow force f to cancel the flow force f of the first item of the right-hand side of the expression (4).
A fourth aspect of the invention is directed to the hydraulic drive unit according to the third aspect of the invention, wherein:
the control valve is provided with a spring for producing a spring force corresponding to the prescribed flow rate;
the second throttle is disposed on the upstream side of the first throttle;
the pressure on the upstream side of the second throttle is applied to the control valve on the side opposite to the spring; and
a pressure on the downstream side of the first throttle is applied to the control valve on the same side as the spring.
The fourth aspect of the invention has the spring 20c on the control valve 20 to apply the pressure P1, which is on the upstream side of the second throttle 42, to the control valve 20 in a direction opposite to the side where the spring 20c is disposed. And, the pressure P3 on the downstream side of the first throttle 41 is applied to the control valve 20 on the same side as that of the spring 20c. Thus, the third expression ((P1xe2x88x92P3)xc2x7A=F+f) holds for the force acting on the control device 20, so that the expression (4) holds accordingly. Thus, the flow force f produced by the control valve 20 is cancelled.
A fifth aspect of the invention is directed to the hydraulic drive unit according to the fourth aspect of the invention, wherein a third throttle is also disposed to adjust the pressure on the upstream side of the second throttle.
According to the fifth aspect of the invention, the third throttle 43 is, for example, disposed to connect the upstream side and the downstream side of the second throttle 42 so as to adjust the pressure P1 on the upstream side of the second throttle 42. Therefore, the pressure P1 on the upstream side of the second throttle 42 can be decreased by appropriately determining the diameter or the like of the third throttle 43. However, the pressure P2 on the downstream side thereof is determined as a lower limit. When the pressure P1 on the upstream side of the second throttle 42 decreases, the force xcex94P12xc2x7A corresponding to the pressure difference xcex94P12 before and after the second throttle 42 can be compensated so as to agree with the flow force f in the expression (4). Thus, the ideal flow control characteristic indicated by LN3 in FIG. 9 can be obtained.
A sixth aspect of the invention is directed to the hydraulic drive unit according to the fifth aspect of the invention, wherein the third throttle is formed in a spool of the control valve.
According to the sixth aspect of the invention, the third throttle 43 is formed in the spool 21 as shown in FIG. 1. Therefore, the third throttle 43 can be easily added to the existing control valve 20, and the production cost can be reduced.
A seventh or eleventh aspect of the invention is directed to the hydraulic drive unit, wherein the hydraulic equipment is a hydraulic motor for driving a fan.
According to the seventh or eleventh aspect of the invention, there is obtained the characteristic L0 that when the engine speed N becomes the prescribed speed Nc or more as shown in FIG. 4, the rotational speed NF of the fan 36 is kept at a prescribed level.
Therefore, according to the seventh or eleventh aspect of the invention, there is an effect that noise of the fan 36 can be suppressed to a prescribed level when the engine speed is at the prescribed level Nc or more.
An eighth or twelfth aspect of the invention is directed to the hydraulic drive unit, wherein the hydraulic equipment is a hydraulic motor, and the control valve and the throttle are built in the hydraulic motor.
According to the eighth or eleventh aspect of the invention, because the control valve 20 and the throttle 41 (42,43) are built within the body 11 of the hydraulic motor 1 as indicated by a dash and dotted line in FIG. 3, an installation area of the hydraulic drive unit becomes small, and the hydraulic drive unit has a simple structure.
A ninth or thirteenth aspect of the invention is directed to the hydraulic drive unit, wherein the hydraulic source is a fixed-capacity hydraulic pump.
The fixed-capacity hydraulic pump 2 such as a gear pump is generally inexpensive as compared with a variable-capacity hydraulic pump and can be used to achieve an ideal flow control characteristic.
A tenth or fourteenth aspect of the invention is directed to the hydraulic drive unit, wherein prescribed flow rate adjustable means which varies the prescribed flow rate is further disposed.
According to the tenth or fourteenth aspect of the invention, the prescribed flow rate of the pressure oil flowing through the first throttle 41 changes when the prescribed spring force of the spring 20c of the control valve 20 changes as shown in FIG. 6. Therefore, when a cooling water temperature of the radiator changes as indicated by t1, t2 and t3 as shown in FIG. 7A, optimum control characteristics L1, L2 and L3 conforming to the cooling water temperatures are obtained.